This invention relates to measurement techniques and, more particularly, to a technique for measuring airfoil deflections on rotating turbomachinery.
In the gas turbine industry, many rapidly advancing technologies are required to support the demands of commercial and military users. Major advances have been attained in recent years in performance, weight minimization, power output, and reliability, as well as the diversity of applications. Recently, aeromechanics, and specifically blade instability prevention, has been identified as a pacing technology, the current growth rate of which presents a restrictive limitation on gas turbine engine development.
One of the major regimes of blade instability which must be avoided is supersonic instability (or flutter), a regime about which blade instability technology has been least developed. This supersonic instability occurs at high Mach numbers representative of contemporary blade designs and is difficult to simulate in simple nonrotating blade cascade tests. Since supersonic instability is expected to present the greatest restrictions to future gas turbine engine fan and compressor development, analysis must progress on representative rotating hardware.
Supersonic instability results from blade dynamics and nonsteady aerodynamic forces which are blade motion dependent. These forces are so coupled that, just beyond the stability boundary, perturbations of blade motion produce net aerodynamic energy input to the blade with increasing amplitude (in the sense of a true feedback system) until nonlinear aerodynamic characteristics intervene to produce a limit cycle response. While considerable progress has been made in theoretically defining supersonic regime, nonsteady aerodynamic flow fields and the corresponding blade motion-dependent forces for nonseparated flow conditions, such theoretical, mathematical solutions are insufficient without precise experimental guidance and confirmation on representative rotating turbomachinery. Since blade instability involves blade dynamics (vibration and flutter)/non-steady aerodynamic interaction over the entire blade span (for which there are yet no transonic solutions), the entire span must be explored experimentally to completely understand and solve the instability problem.
While analysis and understanding of the unsteady dynamic response of blades to unsteady aerodynamic forces is essential to the advancement of gas turbine engine technology, the steady-state (or time averaged deformation of a blade over multiple blade cycles) is also important. Typically, turbomachinery blades are provided with radially variant camber, stagger and lean. Under the influence of centrifugal loading at high rotational speeds, the blades tend to untwist. Since the blade aerodynamic performance is directly related to the orientation of each blade radial section with respect to the motive fluid passing therethrough (incidence angle), such steady-state blade deflections can seriously alter component performance (and perhaps induce nonsteady blade instability as previously discussed). Accordingly, it has become common to factor into each rotating blade a predetermined, theoretically calculated amount of pretwist to compensate for the untwist (or unstagger) anticipated at a particular operating condition. Experimental verification of the anticipated untwist, as well as discovery of any other blade deformation under loading, is therefore necessary for performance optimization. Thus, experimental determination of both the steady-state and dynamic response of blades under actual operating conditions is required.
In the past, several measurement systems have been developed for disposition within a casing surrounding a rotating bladed turbomachinery stage to detect blade passing. One popular scheme incorporated magnetic detectors. Through the appropriate electronic circuitry, deflection of the blade tip was determinable. The limitation of some such systems was that they were necessarily limited to determining only the blade tip deflection in the steady-state mode. Others were limited in accuracy and vibratory deformation range. Clearly, a measurement technique is required to accurately determine the steady-state and dynamic response of rotating blades along their entire blade span.